1. Field of the Invention
The present invention relates to a low voltage drive type electronic appliance containing a power amplifying circuit. Also, the present invention concerns an idle current correcting circuit for correcting an idle current flowing through the power amplifying circuit employed in the low-voltage operable electronic appliance.
2. Description of the Related Art
A power amplifying circuit is a circuit capable of maintaining minimum distortion in an output signal with respect to an input signal by efficiently utilizing a linear operating region of a transistor. To this end, a value of an idle current capable of defining an operating point of the transistor must be kept as an optimum idle current value.
However, there are some cases where idle current values are varied due to adverse influences caused by element deviation. As a consequence, one conventional method has been utilized. That is, a power transistor having superior linearity is selected as the power transistor employed for this type of power amplifier circuit. Furthermore, a high voltage is applied to the selected power transistor so as to secure the necessary high linearity thereof.
Recently, however, voltages of power sources which are available in portable electronic appliances, such as digital portable telephones, are decreasing less and less. Although there is no change in the linearity requirements, the lower power source voltages for the amplifying circuits of these portable electronic appliances are being used.
As a consequence, setting the initial operation values for the idle currents for the element deviation may constitute the major point for securing better linearities for these power amplifying circuits. To set the idle current the following setting methods can be used, for example, the idle current can be fixed by the average value of the element deviation, and the initial idle current can be fixed by individual manual adjustment.
However, the method for manually adjusting the change in the idle current caused by the element deviation of the amplifying circuit requires a lengthy work time and also skilled techniques. Also, this manual adjusting method of the idle current inevitably increases the work steps, which inevitably requires adjustment devices, and therefore is not suitable for mass production. Nevertheless, if the idle current is not changed, then the power consumption is accordingly increased. As a consequence, the idle current must be properly controlled.